Monday, October 3, 2022

How much water should you drink daily?

The amount of water you need to drink daily depends on your age, gender, weather conditions, activity level and many other factors.

I have been hearing since my childhood, which was a long time ago, that one should drink eight glasses of water a day. The reasons sounded very logical: water is needed for getting rid of toxins from the body; it is needed for mucus membranes to stay wet and functional; it is needed for keeping food soft and moving through the intestines, etc.

How do you measure water consumption?

But none of the explanations clarify ‘why eight glasses’? In fact, the right answer is ‘it depends’.

To start with, how big is a glass, anyway? Interchangeably, it is said to be between 200 to 250 mL in quantity. That is a 25% variation.

Second, about 20% of your daily water requirement comes from food. That is another 25% variation if you confuse between how much water you ‘need’ versus how much water you should ‘drink’.

What do official organisations say about water consumption?

Third, the big associations themselves don’t seem to agree:

So if you are trying to get your answer from ‘experts’, well, good luck. Why is there so much variation? Partly because your water needs change based on your age, gender, ethnicity, environment (dry versus humid, hot versus cold), and activity level (outdoors versus indoors). Here is what you should do?

Follow your thirst.

If you feel thirsty, drink. If you don’t, stay away. Unless you are out in some remote mountains with no access to water, it is unlikely that you will face dehydration with no water at hand.

In fact, one of the big problems is if you drink too much water. When your blood has excess water, electrolytes in it become excessively diluted causing problems. Low sodium concentration in blood is particularly life-threatening. The condition is called hyponatremia or low-sodium and needs immediate attention.

Water consumption during exercise

For one of the most dehydrating endurance events, the marathon road race, one was told to drink water before one felt thirsty. Even the American College of Sports Medicine (ACSM) recommended drinking water to prevent decline of body weight of more than 2 percent in a sports activity. But that was in good old days.

More recently, researchers started noticing that more people (including yours truly, a few times) felt sick during such a race due to excessive water ingestion, and not dehydration. It was also found that unless the body weight fell by more than four percent due to water loss, the sports performance did not suffer. So even the marathoners are now advised to drink based on their thirst perception.

If you are sportsperson, hydrate well before the start of your event and drink adequate amounts after the event is over. But there is no need to drink during the event, unless your event is very long, stretching into hours.

Do caffeine and alcohol cause dehydration?

You may be advised by people that tea, coffee, and alcohol also lead to dehydration. Actually, tea and coffee lead to excessive urination but not dehydration enough to cause more fluid loss than what they add (remember that those beverages also provide water).

Alcohol, on the other hand, can lead to dehydration. So don’t drink in excess.

So I hope you have your answers:

  • Drink water based on your thirst;
  • Tea and coffee won’t cause dehydration. So stick to your regular intake;
  • Drink alcohol in moderation, which is a good idea for many other reasons, too;
  • If you spend a lot of time outdoors, especially in dry and warm surroundings, keep a lot of drinking water handy; and
  • If you are given specific hydration instructions by a doctor for a medical conditions, such as in kidney stones, high uric acid or gout, follow that.

And what about that generic advice about 8 to 15 glasses a day? Don’t worry about that. You can’t live life counting glasses.

To Read More:

  1. BBC Future: How much water should you drink a day?
  2. Harvard Medical School: How much water should you drink?
  3. MedicalNewsToday: How much water should you drink a day?
  4. Mayo Clinic: Water: How much should you drink every day?

First published on: 2nd September 2022
Image credit: Andrea Piacquadio on Pexels


    • I don’t think we have such a great mathematical model for calculating it.

      Nearly 10 years ago, I worked with a couple of colleagues to develop such a formula for water intake needed in a marathon, which is obviously an extreme sport. Even there, it turned out that you don’t need to drink too much water because every gram of carbohydrate burned releases 3-4 grams of water. Typically, each gram releases 4 calories, which help an 80 kg person to run 50 meters. So every 1 km that you run, your body releases 60 g of water. In a typical race (and frankly, there is no such thing as typical race because every race has different temperature and wind profile, but on an average in a race, you lose 2 litres per hour. So if you run at 8 km/hour in a race, your body releases 480 mL of water every hour and you are losing 2 litre/hour. So roughly, 1.5 litres per hour is your loss of water in a marathon.

      Now, there is a performance degradation only if you lose more than 4% of your body weight. So you need to lose about 3 kg of weight (for an 80 kg person) in water to notice the performance drop. So even if you do not drink water for the first 2 hours, you should be fine as you are losing only about 1.5 kg/hour in water loss.

      In fact, that is my experience too. You simply do not need any water in a 10 km race, except to wet your parched throat. The body does not need it, your throat needs (you can sip, swirl in your mouth and spit and you will still be all right). In a race of more than 15 km, you are advised to drink 700 mL of water every hour. So after 2 hours, your body is in deficit by 3 litres but you have drunk 1.4 litres. So the net deficit is only 1.6 litres. In effect, you can go for 4 hours at this race without facing any water shortfall. Drink a bit more in the final hour of the marathon.

      So that is the calculation and formula for an extreme sport. You would not need to do anything close to that in normal life. So I don’t think there are formulas for water need in a routine life.


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